Twins provide a number of ways to study gene-environment interaction. One such method is to study monozygotic twins reared apart (MZA). MZAs provide a unique opportunity to study the influence of different environments on identical genotypes. In the Swedish Adoption/Twin Study of Aging, data from 99 pairs of MZAs were tested for interactions between childhood rearing and adult personality (Bergeman et al. 1988). Several significant interactions emerged. In some cases, the environment had a stronger impact on individuals genetically predisposed to be low on a given trait (based on the cotwin’s score). For example, individuals high in extraversion expressed the trait regardless of the environment; however, individuals predisposed to low extraversion had even lower scores in the presence of a controlling family. In other traits, the environment had a greater impact on individuals genetically predisposed to be high on the trait: Individuals predisposed to impulsivity were even more impulsive in a conflictual family environment; individuals low on impulsivity were not affected. Finally, some environments influenced both individuals who were high and low on a given trait, but in opposite directions:
